For the purpose of improving the efficiency of these laser sources, it is common practice to carry out the treatments on the external faces of the emissive cavities. The objective of these treatments is generally to obtain either an antireflection function, i.e. a reflectivity as close as possible to 0%, typically on the output face of the laser cavity, or a high-reflectivity function, i.e. a reflectivity as close as possible to 100%, typically on the face opposite the output face.
Currently, for the purpose of obtaining the desired effect, namely antireflection or high reflectivity, one commonly employed technology consists in depositing thin films of dielectric material on the edge of the laser after the emitting zone has been cleaved. This technology enables the efficiency of these laser sources to be very significantly improved (typically by 250%). From a theoretical standpoint, to obtain the antireflection function on a substrate having a refractive index nsubstrate for a light wave of wavelength λ, it is ideally necessary to deposit on this substrate a film of a material having a refractive index nAR equal to √{square root over (nsubstate)} with a thickness of λ/4nAR. The reflectivity at the air/substrate interface is then theoretically 0%. However, since the number of existing materials is limited, it is very difficult and constricting to obtain a material to be deposited that has the suitable refractive index.
To get round this drawback, it is general practice to deposit multilayer coatings. However, another major drawback associated with this technology remains. It lies in the fact that it is necessary for the linear arrays of lasers to be manipulated directly after cleavage so as to deposit the coatings on each of the external faces of the cavity. Since the size of the chips does not exceed a few millimeters in width for a few millimeters in length, these treatments prove to be complicated. In addition, as regards QCLs, because of the long emission wavelengths of the latter (these sources are used in ranges that may go from the infrared range to the THz range), the thicknesses of the deposited layers rapidly reach critical values, posing problems such as mechanical integrity or induced strain problems. Consequently, the uncertain reliability of these complex treatments is currently putting off their use.
Another technology for adjusting the reflectivity of the external faces of laser cavities consists in etching, on said faces, microstructures that may be smaller than the wavelength in a direction transverse to that in which the light propagates in the cavity. These are then referred to as subwavelength structures. This technology may theoretically enable precise antireflection treatments or high-reflectivity treatments to be carried out. However, it is very difficult at the present time to produce lasers using this technology. This requires producing etched features on a subwavelength scale on cleaved laser cavities, in several stages, so as to obtain an antireflection treatment on the output face and a high-reflectivity treatment on the opposite face. Furthermore, the cleaving carried out, to singularize the laser cavities, does not enable their lengths to be guaranteed. Consequently, the fabrication processes for such lasers is at the present time complex, costly and of uncertain quality.
To summarize, the existing technologies for improving the efficiency of light emitters, such as laser sources for example, notably by means of antireflection treatments, are difficult to implement. In addition, their performance and their reliability are not guaranteed.
For the purpose of alleviating the aforementioned drawbacks, the invention provides a process for fabricating light-emitting devices of improved efficiency, which manufacturing process may be a wafer-scale process.
More precisely, the invention is based on the exploitation of artificial materials of adjustable refractive index, these artificial materials being obtained by etching subwavelength structures, as explained above. The invention makes it possible to fabricate, notably using a wafer-scale process, laser cavities the external faces of which have precise antireflection treatments or high-reflectivity treatments, resulting in the production of optimized light emitters.